BepiColombo is Now Endangered by a Lack of Thrust

On April 26, the ESA/JAXA BepiColombo probe began its next maneuver but the power unit of the spacecraft failed to deliver enough electrical power to feed the propulsion system properly. Some problems with the spacecraft’s Transfer Module are preventing the thrusters from working at full power.

A combined team from ESA and the mission’s industrial partners worked on the problem and by 7 May were able to restore 90% of the thrust the probe was generating before the issue. If the current power level is maintained BepiColombo should arrive at Mercury in time for its fourth gravity assist at the planet in September this year. 

However, the Bepicolombo Flight Control Team working at ESA’s ESOC mission center in Darmstadt, Germany, has arranged additional ground station passes to estimate how the malfunction will affect upcoming maneuvers and to identify the root cause of the issue and eventually restore the full thrust.

Mission Overview

BepiColombo is a joint mission between ESA and the Japan Aerospace Exploration Agency (JAXA) launched on board an Ariane 5 vector from the Centre Spatial Guyanais, Kourou, on October 20, 2018. It’s the most advanced mission to Mercury, the least explored planet of the inner Solar System.

The program aims to answer questions and solve many mysteries about the small planet, such as the origin of ice in the polar craters or the existence of a magnetic field. This investigation will be conducted by the two “scientific laboratories” carried by the Mercury Transfer Module.

The probe is composed of three modules: the Mercury Transfer Module, the Mercury Planetary Orbiter, and the Mercury Magnetospheric Orbiter. The first one is used to transport the other two which ESA and JAXA respectively developed. Once in orbit, they will separate to make scientific measurements from different distances. 

The mission foresees several planet flybys and it’s expected to arrive at Mercury on December 5th, 2025. The routine science operations will begin in April 2026.

The most advanced electric propulsion system

The Solar Electric Propulsion System that BepiColombo relies on to reach the innermost planet is the most powerful and highest-performance electric propulsion system flown on any space mission to date. The key components of the system are the ion thrusters with their fuel tanks and the power unit that delivers electrical power generated by 15-meter-long solar arrays, located in the Mercury Transfer Module.

BepiColombo power conditioning and distribution relies on a powerful PCDU designed by Terma. This unit supplies the ion propulsion system and other functions during the seven-year cruise phase to Mercury. 

The high performance of T6s thrusters developed by QinetiQ, in terms of fuel consumption, is critical. Inert xenon gas is fed into the thrusters, where electrons are first stripped off the xenon atoms. The resulting electrically charged atoms, referred to as ions, are then focused and ejected out of the thrusters using a high-voltage grid system at a velocity of 50,000 meters per second.

This exhaust velocity is 15 times greater than conventional chemical rocket thrusters and allows a dramatic reduction in the amount of propellant required to achieve the mission.

At full power, the engines should be able to generate a constant thrust of 250 mN, the equivalent weight of three 1 euro coins, meaning that the thrusters have to keep firing for long periods to be effective. But in the absence of any drag the maneuvers that are possible and the payload that can be carried are dramatic.

Furthermore, they guarantee extreme precision since every T6 thruster comes with a gimbal system developed by RUAG Space that makes it maneuverable.